This invention relates to anti-photocopying and anti-tele-facsimile paper, that is to say, paper which when carrying information in conventional black or similar dark color cannot be readily photocopied or transmitted by telefacsimile in a visually readable manner.
The present day availability of improved photocopiers has increased the problem of rendering documents or portions thereof resistant to photocopying in a readable manner. Anti-photocopying paper which is successful in preventing visually readable photocopying by most present day photocopiers is described in U.S. Pat. No. 4,522,429 (Gardner et al) issued Jun. 11, 1985, U.S. Pat. No. 4,632,429 (Gardner et al) issued Dec. 30, 1986, and U.S. Pat. 4,867,481 (Gundjian) issued Sept. 19, 1989, generally referred to hereinafter as Nocopi technology.
U.S. Pat. No. 4,522,429 teaches the use of anti-photocopying paper having a color with a reflection spectral response of less than about 10% for light with a wavelength below about 600 millimicrons and yet which is sufficiently visually contrasting with information, when such information is typed thereon or otherwise applied thereto, to enable such information to be read by the human eye when the paper is viewed under white light.
U.S. Pat. No. 4,632,429 teaches the use of anti-photocopying paper with a front face having a color with a reflection spectral response which is effectively zero for light with a wavelength below about 625 millimicrons and less than about 1% up to about 1,000 millimicrons so as to render the paper substantially incapable of being photocopied in an information readable manner, after substantially non-translucent information has been typed or otherwise applied to the front face, the paper being capable of transmitting visible light from a rear face to the front face to cause sufficient contrast between the substantially non-translucent information and the transmitted light to enable the information to be read by a human eye viewing the front face of the paper when visible light is transmitted to the paper from the rear face to the front face thereof.
Further improvement in the anti-photocopying and anti-tele-facsimile effect is achieved by the teachings of European patent application 88301745.1, by using spatial spectral modulation of the paper reflectance at a specific single or preferably multiple frequencies.